Nanofiber and nonwoven fabric

ABSTRACT

An object of the invention is to provide a nanofiber having excellent uniformity of the fiber diameter and capable of giving a satisfactory external appearance in a case in which the nanofiber is used to produce a nonwoven fabric, and a nonwoven fabric produced using the nanofiber. The nanofiber of the invention is a nanofiber containing a cellulose acylate having a degree of substitution of from 2.75 to 2.95.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No. PCT/JP2017/002554 filed on Jan. 25, 2017, which claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2016-012717 filed on Jan. 26, 2016. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a nanofiber produced using a cellulose acylate and to a nonwoven fabric.

2. Description of the Related Art

Nanofibers, that is, fibers having a diameter in the order of nanometers, such as several nanometers or larger and smaller than 1,000 nm, are utilized as materials for manufactured products such as biofilters, sensors, fuel cell electrode materials, precision filters, and electronic paper. Thus, development of use applications in various fields such as engineering and medicine is in active progress.

For example, in JP2009-291754A, “a harmful substance removal material which consists of a carrier constituted of fibers, in which the fiber diameter is from 10 nm to 1 μm, and the pore diameter of the carrier is from 100 μm to 1 mm” is described ([Claim 1]), and a fiber containing a cellulose ester as a main component or a cellulose acylate fiber is described as the fiber that constitutes the carrier ([Claim 3] and paragraphs [0019] to [0021]).

SUMMARY OF THE INVENTION

The inventors of the present invention conducted an investigation on nanofibers produced using a cellulose acylate, and the inventors found that depending on the type of the cellulose acylate used, uniformity of the fiber diameter of a nanofiber to be produced may become inferior, and the external appearance may be poor in a case in which the nanofiber is used to produce a nonwoven fabric.

Thus, an object of the invention is to provide a nanofiber that has excellent uniformity of the fiber diameter and gives a satisfactory external appearance in a case in which the nanofiber is used to produce a nonwoven fabric, and a nonwoven fabric produced using the nanofiber.

The inventors conducted a thorough investigation in order to achieve the object described above, and as a result, the inventors found that a nanofiber produced by using a cellulose acylate having a particular degree of substitution has excellent uniformity of the fiber diameter and gives a satisfactory external appearance in a case in which the nanofiber is used to produce a nonwoven fabric, thus completing the invention.

That is, the inventors found that the above-described object can be achieved by the following configuration.

[1] A nanofiber comprising a cellulose acylate having a degree of substitution that satisfies Formula (1):

2.75≤Degree of substitution≤2.95  (1)

[2] The nanofiber according to [1], in which a proportion of an average fiber length with respect to an average fiber diameter is 1,000 or higher.

[3] The nanofiber according to [1] or [2], in which an average fiber diameter is 50 to 800 nm.

[4] The nanofiber according to any one of [1] to [3], in which an average fiber length is 500 μm or more.

[5] The nanofiber according to any one of [1] to [4], in which an acyl group carried by the cellulose acylate is an acetyl group.

[6] The nanofiber according to any one of [1] to [5], in which a hemicellulose amount of the cellulose acylate is 0.1% to 3.0% by mass.

[7] The nanofiber according to any one of [1] to [6], in which a viscosity of a solution obtained by dissolving the nanofiber in dichloromethane at a concentration of 6% by mass is 300 mPa·s or higher.

[8] A nonwoven fabric comprising the nanofiber according to any one of [1] to [7].

[9] The nonwoven fabric according to [8], in which the nonwoven fabric is used for a medical filter or a face mask.

According to the invention, a nanofiber having excellent uniformity of the fiber diameter and capable of giving a satisfactory external appearance in a case in which the nanofiber is used to produce a nonwoven fabric, and a nonwoven fabric produced using the nanofiber can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a nanofiber producing apparatus.

FIG. 2 is a cross-sectional view illustrating the tip of a nozzle.

FIG. 3 is a scanning electron microscope (SEM) image (magnification ratio: 1,800 times) of a nonwoven fabric formed from the nanofiber produced in Example 1.

FIG. 4 is an SEM image (magnification ratio: 1,800 times) of a nonwoven fabric formed from the nanofiber produced in Example 2.

FIG. 5 is an SEM image (magnification ratio: 1,800 times) of a nonwoven fabric formed from the nanofiber produced in Comparative Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the invention will be described in detail.

The explanation of the configuration requirements described below is based on representative embodiments of the invention; however, the invention is not intended to be limited to such embodiments.

According to the present specification, a numerical value range indicated using “to” means a range including the numerical values described before and after “to” as the lower limit and the upper limit.

[Nanofiber]

The nanofiber of the invention is a nanofiber containing a cellulose acylate having a degree of substitution that satisfies Formula (1):

2.75≤Degree of substitution≤2.95  (1)

Here, the term “nanofiber” according to the present specification means a fiber having an average fiber diameter of from 10 nm to 1,000 nm as measured by the measurement method described below.

<Average Fiber Diameter>

The average fiber diameter means a value measured as follows.

The surface of a nonwoven fabric formed from a nanofiber is observed by taking a Transmission Electron Microscope (TEM) image or a Scanning Electron Microscope (SEM) image.

An observation based on an electron microscopic image is performed at a magnification ratio selected from 1,000 times to 5,000 times depending on the size of the constituent fiber. However, the sample, the observation conditions, and the magnification ratio are adjusted so as to satisfy the following conditions.

(1) One straight line X is drawn at an arbitrary site within an image to be observed, and 20 or more fibers intersect this straight line X.

(2) A straight line Y perpendicularly intersecting the straight line X is drawn in the same image, and 20 or more fibers intersect the straight line Y.

In regard to an electron microscopic observation image such as described above, for each of the fibers intersecting the straight line X and the fibers intersecting the straight line Y, at least 20 values (that is, at least 40 values in total) of the width (minor axis of the fiber) are read out. In this manner, an observation of at least 3 sets or more of electron microscopic images such as described above is made, and at least 40×3 sets (that is, at least 120) fiber diameters are read out.

An average fiber diameter is determined by averaging the fiber diameters read out as such.

<Average Fiber Length>

The average fiber length of a cellulose fiber refers to a value measured as follows.

That is, the fiber length of the cellulose fiber can be determined by analyzing the electron microscopic observation image used on the occasion of measuring the average fiber diameter described above.

Specifically, in an electron microscopic observation image such as described above, for each of the fibers intersecting the straight line X and the fibers intersecting the straight line Y, at least 20 values (that is, at least 40 values in total) of the fiber length are read out.

Thus, an observation of at least 3 sets or more of electron microscopic images such as described above is made, and at least 40 values×3 sets (that is, at least 120 values) of the fiber length are read out.

The average fiber length is determined by averaging the fiber lengths thus read out.

Since the nanofiber of the invention contains a cellulose acylate having a degree of substitution of from 2.75 to 2.95 as described above, the nanofiber has excellent uniformity of the fiber diameter and gives a satisfactory external appearance in a case in which the nanofiber is used to produce a nonwoven fabric.

The reason why such effects are provided is not clearly known in detail; however, the inventors of the present invention speculate the reason as follows.

That is, according to this invention, it is considered that in a case in which a nanofiber is produced by utilizing an electric field spinning method (hereinafter, also referred to as “electrospinning method”), by using a cellulose acylate having a degree of substitution of from 2.75 to 2.95, the crystallinity of the cellulose acylate is increased, thereby spinning of the cellulose acylate in a liquid droplet state is suppressed, and entanglement of the cellulose acylate molecules is promoted.

In regard to the nanofiber of the invention, for the reason that a single nonwoven fabric composed of the nanofiber can be easily produced, the proportion of the average fiber length with respect to the average fiber diameter, that is, the aspect ratio (average fiber length/average fiber diameter) is preferably 1,000 or larger, more preferably 2,500 to 20,000, and particularly preferably 5,000 to 20,000.

In regard to the nanofiber of the invention, for the reason that the fiber has high mechanical strength and a nonwoven fabric can be produced easily, the average fiber diameter is preferably 50 to 800 nm, and more preferably 100 to 600 nm. Furthermore, in a case in which the average fiber diameter is 50 to 800 nm, effects such as a size effect, a supramolecular arrangement effect, a cell recognition effect, and a hierarchical structure effect can also be expected.

Furthermore, in regard to the nanofiber of the invention, for the reason that fraying of the fiber is suppressed in a case in which a nonwoven fabric is formed, the average fiber length is preferably 500 μm or more, more preferably 1 mm or more, and even more preferably 1.5 to 5 mm.

In regard to the nanofiber of the invention, for the reason that uniformity of the fiber diameter is further enhanced, and a more satisfactory external appearance is obtained in a case in which the nanofiber is used to produce a nonwoven fabric, the viscosity of a solution obtained by dissolving the nanofiber in dichloromethane at a concentration of 6% by mass (hereinafter, also referred to as “6% solution viscosity”) is preferably 300 mPa·s or higher, more preferably 300 to 1,000 Pa·s, even more preferably 300 to 900 mPa·s, and particularly preferably 350 to 800 mPa·s.

It is speculated that such effects are obtained because in a case in which the nanofiber is produced by utilizing an electrospinning method, spinning of the nanofiber in a liquid droplet state can be suppressed, and skinning of the nozzle can be suppressed. Particularly, the present inventors assume the reason why the uniformity of the fiber diameter is enhanced by controlling the degree of substitution of cellulose acylate and the 6% solution viscosity, as follows.

First, in order to form a uniform nanofiber, it is considered important to form entanglement of polymer molecules so that the nanofiber is not frayed during the course in which a solution is discharged from a nozzle and is dried. Also, the inventors speculated that regarding a method for controlling the entanglement of polymer molecules, (a) a method of strengthening the intermolecular interaction (crystallinity) (hereinafter, simply referred to as “method (a)” in the present paragraph) and (b) a method of lengthening the length (molecular weight) of the molecule (hereinafter, simply referred to as “method (b)” in the present paragraph) are useful. Therefore, in the present invention, the degree of substitution of cellulose acylate is adjusted in order to perform method (a), and the 6% solution viscosity is adjusted in order to perform method (b). Particularly, the adjustment of the degree of substitution of cellulose acylate suppresses rapid formation of entanglement in the later stage of drying, and the adjustment of the 6% solution viscosity controls the formation of entanglement in the early stage of drying. Thus, since entanglement can be controlled over the entire process, it is speculated that spinning of the nanofiber in a liquid droplet state is suppressed, and a uniform nanofiber can be produced.

Meanwhile, in the present specification, the 6% solution viscosity refers to a value measured by the following procedure.

First, a solution is obtained by precisely weighing dried cellulose acylate and dissolving the cellulose acylate at a concentration of 6% by mass in a mixed solvent of dichloromethane and methanol at a mass ratio of 91:9, and the flow time at 25° C. of the solution is measured using an Ostwald viscometer, and the 6% solution viscosity is calculated by the following formula.

6% solution viscosity (mPa·s)=Time for flow-down (seconds)×coefficient of viscometer

Here, the coefficient of viscometer can be determined by measuring the number of seconds for flow-down by an operation similar to the operation for the solution, using a standard liquid for viscometer calibration. Specifically, the coefficient of viscometer can be determined by the formula: coefficient of viscometer=absolute viscosity (cps) of standard liquid×density (1.235 g/cm³) of solution/density (g/cm³) of standard liquid/time (seconds) for flow-down of standard liquid.

In the following description, the cellulose acylate included in the nanofiber of the invention, a method for synthesizing the cellulose acylate, and a method for producing the nanofiber of the invention will be explained in detail.

[Cellulose Acylate]

The cellulose acylate included in the nanofiber of the invention is a cellulose acylate having a degree of substitution that satisfies Formula (1):

2.75≤Degree of substitution≤2.95  (1)

Here, the “cellulose acylate” refers to a cellulose ester in which some or all of the hydrogen atoms that constitute hydroxyl groups of cellulose, that is, free hydroxyl groups existing at the 2-position, 3-position, and 6-position of a β-1,4-bonded glucose unit, have been substituted by acyl groups.

Furthermore, the “degree of substitution” refers to the degree of substitution of the hydrogen atoms that constitute hydroxyl groups of cellulose by acyl groups, and the degree of substitution can be calculated by comparing the area intensity ratio of carbon atoms of cellulose acylate measured by a ¹³C-NMR method.

<Substituent (Acyl Group)>

Specific examples of the acyl group include an acetyl group, a propionyl group, and a butyryl group.

The acyl groups to be substituted may be composed only of a single kind (for example, only an acetyl group) or may be of two or more kinds.

According to the invention, for the reason that the uniformity of the fiber diameter is further enhanced and a more satisfactory external appearance is obtained in a case in which the nanofiber is used to produce a nonwoven fabric, in the case of using one kind of acyl group, it is preferable that the acyl group is an acetyl group; and in the case of using two or more kinds of acyl groups, it is preferable that one kind of the acyl groups is an acetyl group. Above all, an embodiment in which one kind of acyl group is used, and the acyl group is an acetyl group, is preferred.

<Degree of Substitution>

The degree of substitution of the acyl group is 2.75 to 2.95 as described above; however, for the reason that the uniformity of the fiber diameter is further enhanced and a more satisfactory external appearance is obtained in a case in which the nanofiber is used to produce a nonwoven fabric, the degree of substitution is preferably 2.80 to 2.95, and more preferably 2.88 to 2.95.

The method for adjusting the degree of substitution will be described in detail in the method for synthesizing cellulose acylate that will be described below.

<Hemicellulose Amount>

According to the invention, for the reason that the uniformity of the fiber diameter is further enhanced and a more satisfactory external appearance is obtained in a case in which a nonwoven fabric is produced, the hemicellulose amount of the cellulose acylate is preferably 0.1% to 3.0% by mass, and more preferably 0.1% to 2.0% by mass.

It is considered that such effects are obtained because in a case in which a nanofiber is produced by utilizing an electrospinning method, crystallinity of the cellulose acylate is increased, and thereby spinning of the nanofiber in a liquid droplet state is suppressed.

According to the present specification, the hemicellulose amount refers to a value calculated from a sugar analysis based on an alditol acetate method (Borchadt, L. G.; Piper, C. V.; Tappi, 53, 257-260 (1970)).

Furthermore, the method for adjusting the hemicellulose amount will be described in detail in the section for a method for synthesizing cellulose acylate that will be described below.

<Molecular Weight>

The number average molecular weight (Mn) of the cellulose acylate included in the nanofiber of the invention is not particularly limited; however, from the viewpoint of the mechanical strength of the nanofiber, the number average molecular weight is preferably 40,000 or more, more preferably 40,000 to 150,000, and even more preferably 60,000 to 100,000.

The weight-average molecular weight (Mw) of the cellulose acylate is not particularly limited; however, from the viewpoint of the mechanical strength of the nanofiber, the weight-average molecular weight is preferably 100,000 or more, more preferably 100,000 to 500,000, and even more preferably 150,000 to 300,000.

The weight-average molecular weight or number average molecular weight according to the present specification is a value measured by a gel permeation chromatography (GPC) method under the following conditions.

-   -   Apparatus name: HLC-8220 GPC (Tosoh Corporation)     -   Type of column: TSK gel Super HZ4000 and HZ2000 (Tosoh         Corporation)     -   Eluent: Dimethylformamide (DMF)     -   Flow rate: 1 ml/min     -   Detector: Refractive index (RI)     -   Sample concentration: 0.5%     -   Calibration curve base resin: TSK standard polystyrene         (molecular weights 1050, 5970, 18100, 37900, 190000, and 706000)

The content of the cellulose acylate in the nanofiber of the invention is not particularly limited; however, the content is preferably 25% by mass or more, more preferably 40% to 100% by mass, and even more preferably 60% by mass to 100% by mass, with respect to the total mass of the nanofiber.

[Method for Synthesizing Cellulose Acylate]

Regarding a method for synthesizing the cellulose acylate as described above, the description in Hatsumei Suishin Kyokai Kokai Giho (Journal of Technical Disclosure) (Technology No. 2001-1745, published on Mar. 15, 2001, Japan Institute for Promoting Invention and Innovation), p. 7 to 12 is also applicable.

<Raw Materials>

Regarding the raw material for cellulose, suitable examples include raw materials originating from hardwood pulp, softwood pulp, and cotton linter. Among them, raw materials originating from cotton linter are preferred because the hemicellulose amount is small, and a nanofiber having further enhanced uniformity of the fiber diameter can be produced.

<Hemicellulose Amount>

Adjustment of the hemicellulose amount can be adjusted by purifying a raw material of cellulose by an appropriate method.

For example, the hemicellulose amount can be adjusted by subjecting a raw material of cellulose to a purification bleaching process combining treatments such as a digestion treatment based on a sulfite pulping method or a kraft cooking method; a bleaching treatment based on an oxygen-based or chlorine-based bleaching agent; and an alkali purification treatment.

Specifically, in regard to the purification bleaching process combining the digestion treatment, bleaching treatment, and alkali purification treatment, a method of performing a purification treatment at a low temperature of 20° C. to 40° C. using a strongly alkaline aqueous solution at a concentration of 3% to 25% by mass on the occasion of applying the alkali purification treatment may be suitably used.

<Activation>

It is preferable that the raw material of cellulose is subjected to a treatment of contacting with an activating agent (activation), prior to acylation.

Specific examples of the activating agent include acetic acid, propionic acid, and butyric acid, and among them, acetic acid is preferred.

The amount of addition of the activating agent is preferably 5% to 10,000%, more preferably 10% to 2,000%, and even more preferably 30% to 1,000%.

The method for addition can be selected from methods such as spraying, dropwise addition, and immersion.

The activation time is preferably 20 minutes to 72 hours, and more preferably 20 minutes to 12 hours.

The activation temperature is preferably 0° C. to 90° C., and more preferably 20° C. to 60° C.

Furthermore, an acylation catalyst such as sulfuric acid may be added to the activating agent, in an amount of 0.1% to 10% by mass.

<Acylation>

It is preferable for synthesizing a uniform cellulose acylate that the hydroxyl groups of cellulose are acylated by reacting cellulose with an acid anhydride of a carboxylic acid using Brønsted acid or a Lewis acid (see “Rikagaku Shoten (Dictionary of Physics and Chemistry”, 5^(th) Edition (2000)) as a catalyst, and control of the molecular weight is also enabled.

Examples of the method for obtaining a cellulose acylate include a method of causing a reaction by adding two kinds of carboxylic acid anhydrides as acylating agents as a mixture or in sequence to the system; a method of using a mixed acid anhydride of two kinds of carboxylic acids (for example, a mixed acid anhydride of acetic acid and propionic acid); a method of forming a mixed acid anhydride (for example, a mixed acid anhydride of acetic acid and propionic acid) within the reaction system by using acid anhydrides of a carboxylic acid and another carboxylic acid (for example, acid anhydrides of acetic acid and propionic acid) as raw materials, and reacting the mixed acid anhydride with cellulose; and a method of first synthesizing a cellulose acylate having a degree of substitution of less than 3, and further acylating residual hydroxyl groups by using an acid anhydride or an acid halide.

Furthermore, in regard to the synthesis of a cellulose acylate having a high degree of the 6-position substitution, the details are described in JP1999-5851A (JP-H11-5851A), JP2002-212338A, JP2002-338601A, and the like.

(Acid Anhydride)

The acid anhydride of a carboxylic acid is preferably an acid anhydride of a carboxylic acid having 2 to 6 carbon atoms, and specifically, suitable examples include acetic anhydride, propionic anhydride, and butyric anhydride.

It is preferable that the acid anhydride is added in an amount of 1.1 to 50 equivalents, more preferably 1.2 to 30 equivalents, and even more preferably 1.5 to 10 equivalents, with respect to the hydroxyl groups of cellulose.

(Catalyst)

Regarding the acylation catalyst, it is preferable to use a Brønsted acid or a Lewis acid, and it is more preferable to use sulfuric acid or perchloric acid.

The amount of addition of the acylation catalyst is preferably 0.1% to 30% by mass, more preferably 1% to 15% by mass, and even more preferably 3% to 12% by mass.

(Solvent)

Regarding the acylation solvent, it is preferable to use a carboxylic acid, and it is more preferable to use a carboxylic acid having from 2 to 7 carbon atoms. Specifically, it is even more preferable to use, for example, acetic acid, propionic acid, or butyric acid. These solvents may also be used as mixtures.

(Conditions)

In order to control a temperature increase caused by the heat of reaction of acylation, it is preferable that the acylating agent is cooled in advance.

The acylation temperature is preferably −50° C. to 50° C., more preferably −30° C. to 40° C., and even more preferably −20° C. to 35° C.

The minimum temperature of the reaction is preferably −50° C. or higher, more preferably −30° C. or higher, and even more preferably −20° C. or higher.

The acylation time is preferably 0.5 hours to 24 hours, more preferably 1 hour to 12 hours, and even more preferably 1.5 hours to 10 hours.

Adjustment of the molecular weight is enabled by controlling the acylation time.

(Reaction Terminating Agent)

It is preferable that a reaction terminating agent is added after the acylation reaction.

The reaction terminating agent may be any compound capable of decomposing an acid anhydride, and specific examples include water, an alcohol having 1 to 3 carbon atoms, and a carboxylic acid (for example, acetic acid, propionic acid, or butyric acid). Above all, a mixture of water and a carboxylic acid (acetic acid) is preferred.

The composition of water and the carboxylic acid is such that the content of water is preferably 5% to 80% by mass, more preferably 10% to 60% by mass, and even more preferably 15% to 50% by mass.

(Neutralizing Agent)

After termination of the acylation reaction, a neutralizing agent may be added.

Examples of the neutralizing agent include ammonium, organic quaternary ammoniums, alkali metals, metals of Group 2, metals of Groups 3 to 12, and carbonates, hydrogen carbonates, organic acid salts, hydroxides, or oxides of the elements of Groups 13 to 15. Specifically, suitable examples include carbonate, hydrogen carbonate, acetate, or hydroxide of sodium, potassium, magnesium, or calcium.

(Partial Hydrolysis)

The cellulose acylate obtained by the acylation described above has a total degree of substitution of almost 3; however, for the purpose of adjusting the degree of substitution to a desired value (for example, about 2.8), the degree of acyl substitution of the cellulose acylate can be decreased to a desired extent, by partially hydrolyzing ester bonds by maintaining the cellulose acylate for several minutes to several days at 20° C. to 90° C. in the presence of water and a small amount of catalyst (for example, an acylation catalyst such as residual sulfuric acid). Meanwhile, partial hydrolysis can be terminated as appropriate using residual catalyst and the neutralizing agent.

(Filtration)

Filtration may be carried out in any step between the completion of acylation and reprecipitation. It is also preferable to dilute the system with an appropriate solvent prior to filtration.

(Reprecipitation)

The cellulose acylate solution can be mixed with water or an aqueous solution of a carboxylic acid (for example, acetic acid or propionic acid), and thus reprecipitation can be induced. Reprecipitation may be any of continuous type or batch type.

(Washing)

After reprecipitation, it is preferable to perform a washing treatment. Washing is carried out using water or warm water, and completion of washing can be checked through the pH, ion concentration, electrical conductivity, elemental analysis, or the like.

(Stabilization)

It is preferable that a weak alkali (carbonate, hydrogen carbonate, hydroxide, or oxide of Na, K, Ca, Mg, or the like) is added to the cellulose acylate obtained after washing, for the purpose of stabilization.

(Drying)

It is preferable that the cellulose acylate is dried at 50° C. to 160° C. until the percentage water content reaches 2% by mass or less.

[Method for Producing Nanofiber]

There are no particular limitations on the method for producing the nanofiber of the invention; however, the nanofiber can be produced by, for example, discharging a solution obtained by dissolving the above-described cellulose acylate in a solvent, through a nozzle at a constant temperature in the range of from 5° C. to 40° C., applying a voltage between the solution and a collector, and jetting out fibers from the solution into the collector. The details will be described below with reference to the drawings.

A nanofiber producing apparatus 110 illustrated in FIG. 1 is intended for producing a nanofiber 46 from a solution 25 containing cellulose acylate dissolved in a solvent. The nanofiber producing apparatus 110 includes a spinning chamber 111, a solution supply unit 112, a nozzle 13, an accumulation unit 15, and a power supply 65. The spinning chamber 111 is configured to accommodate, for example, the nozzle 13, a portion of the accumulation unit 15, and the like and to be tightly sealed, so that leakage of the solvent gas to the outside is prevented. The solvent gas is vaporized solvent of the solution 25.

The solvent may be a single substance or may be a mixture composed of a plurality of compounds. Examples of the solvent that dissolves cellulose acylate include methanol, ethanol, isopropanol, butanol, benzyl alcohol, acetone, methyl ethyl ketone, cyclohexanone, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl formate, ethyl formate, hexane, cyclohexane, dichloromethane, chloroform, carbon tetrachloride, benzene, toluene, xylene, dimethylformamide, N-methylpyrrolidone (NMP), diethyl ether, dioxane, tetrahydrofuran, and 1-methoxy-2-propanol. These may be used singly or may be used as mixtures, in accordance with the type of the polymer, the saturation vapor pressure Ps, the viscosity of the solution 25, and the like. According to the present embodiment, a mixture of dichloromethane and NMP, a mixture of dichloromethane and cyclohexanone, a mixture of acetone and cyclohexanone, or the like is used as the solvent.

In the upper part of the spinning chamber 111, the nozzle 13 is disposed. The nozzle 13 is intended for discharging the solution 25 in a state of being charged to a first polarity by means of the power supply 65 as will be described below. As illustrated in FIG. 2, the nozzle 13 is configured as a cylinder, and the solution 25 is discharged through an opening at the tip (hereinafter, simply referred to as “tip opening”) 13 a. The tip opening 13 a is an outlet for the solution 25. The nozzle 13 is, for example, made of stainless steel and has an outer diameter of 0.65 mm and an inner diameter of 0.4 mm, and a verge of tip opening 13 b around the tip opening 13 a is cut so as to perpendicularly intersect the pipe core direction. The verge of tip opening 13 b, which is this cut surface, is polished to become flat.

The material of the nozzle 13 may be composed of an electrically conductive material such as, for example, an aluminum alloy, a copper alloy, or a titanium alloy, instead of stainless steel. For electrospinning, it is desirable that the solution 25 is brought into contact with metal members at several sites, voltage is applied, and the solution 25 is discharged through the tip opening 13 a in a state of being charged to the first polarity. Therefore, as long as a voltage is applied at several sites through the passage to the tip opening 13 a, and the solution 25 is charged to the first polarity at the time of being discharged through the tip opening 13 a, it is not necessarily essential that the tip opening 13 a is made of an electrically conductive material.

As illustrated in FIG. 1, the base end of the nozzle 13 is connected to a pipe 32 of the solution supply unit 112. The solution supply unit 112 is intended to supply the solution 25 to the nozzle 13 of the spinning chamber 111. The solution supply unit 112 includes a storage container 30, a first temperature regulator 133, a pump 31, and the pipe 32. The storage container 30 stores the solution 25. The first temperature regulator 133 regulates the temperature of the solution 25 that is in storage, by means of the storage container 30.

The pump 31 sends the solution 25 from the storage container 30 to the nozzle 13 through the pipe 32. The flow rate of the solution 25 that is sent out through the nozzle 13 can be regulated by changing the speed of rotation of the pump 31. According to the present embodiment, the flow rate of the solution 25 is set to 3 cm³/hour; however, the flow rate is not limited to this. By having the solution 25 sent to the nozzle 13 by the pump 31, the solution 25 comes out through the tip opening 13 a.

The solution 25 in the storage container 30 is such that the saturation vapor pressure Ps (unit: kPa) of the solvent and the concentration C (unit: g/100 cm³) of the cellulose acylate satisfy the following condition (1). While being in a state of satisfying this condition (1), the solution 25 is sent to the nozzle 13 and comes out through the tip opening 13 a. In the present embodiment, temperature regulators (not shown in the diagram) are provided at the pipe 32 and the nozzle 13 so that the solution 25 in a state of satisfying the condition (1) is guided from the storage container 30 to the tip opening 13 a and comes out through the tip opening 13 a. Thus, in a state of having the temperature of the solution 25 maintained at the temperature in the storage container 30 by these temperature regulators, the solution 25 is guided to the nozzle 13 and is discharged through the tip opening 13 a.

Ps×C≤300  (1)

The saturation vapor pressure Ps(t) of the solvent at a temperature t can be determined by the following Formula (2). Here, the number of components of the solvent is designated as n (n represents a natural number of 1 or greater); the saturation vapor pressure of a single substance of component i (i represents a natural number of from 1 to n) at the temperature t is designated as Pi(t); and the molar fraction of the component i in the solvent is designated as Xi. In a case in which the number of components is n, the saturation vapor pressure Ps(t) is defined by the following formula. Ps under the above-described condition (1) is determined by defining the temperature of the solution 25 coming out through the nozzle 13 as temperature t in Formula (2). In the present embodiment, since the temperature of the solution 25 is maintained constant through the passage from the storage container 30 to the tip opening 13 a, the temperature at the storage container 30 is used as temperature t of Formula (2), and thereby the saturation vapor pressure Ps is determined. Furthermore, regarding the concentration C, in a case in which the volume of the solution 25 is designated as V (unit: cm³) and the mass of the cellulose acylate is designated as M (unit: g), the concentration C is determined by the formula: (M×100)/V.

It is preferable that the saturation vapor pressure Ps is in the range of from 10 kPa to 50 kPa. In a case in which the saturation vapor pressure Ps is 10 kPa or higher, the solvent can easily evaporate compared to the case in which the saturation vapor pressure Ps is less than 10 kPa. Therefore, ball-shaped liquid droplets of the solution 25 or particles of solid components are not generated. Furthermore, in a case in which the saturation vapor pressure is 50 kPa or less, since the solvent does not easily evaporate compared to the case in which the saturation vapor pressure is higher than 50 kPa, solidification of the solution 25 caused by drying is suppressed.

The first temperature regulator 133 regulates the saturation vapor pressure Ps of the solvent in the solution 25 by regulating the temperature of the solution 25. Meanwhile, the saturation vapor pressure Ps can be regulated by, in replacement of or in addition to the regulation of the temperature of the solution 25, using a mixture of a plurality of compounds as the solvent of the solution 25 and changing the mixing ratio of the compounds.

The temperature of the solution 25 coming out through the nozzle 13 is preferably in the range of from 5° C. to 40° C., and in the present embodiment, the temperature of the solution 25 is set to 25° C.±1° C. (in the range of from 24° C. to 26° C.). In order to adjust the temperature of the solution 25 coming out through the nozzle 13 to the range described above, it is preferable that the solution 25 is stored in the storage container 30 after the temperature of the solution is regulated to the range of from 5° C. to 40° C., and in the present embodiment, the temperature is adjusted to 25° C.±1° C. In a case in which the temperature of the solution 25 is 5° C. or higher, the solution 25 does not easily undergo gelation caused by low temperature, compared to the case in which the temperature is below 5° C., and the solution 25 comes out stably through the nozzle 13. Furthermore, in a case in which the temperature of the solution 25 is 40° C. or lower, vigorous evaporation (flash) caused by the temperature of the solvent increasing above the boiling point does not easily occur, compared to the case in which the temperature is higher than 40° C., and solidification of the solution 25 caused by drying is suppressed. The temperature of the solution 25 coming out through the nozzle 13 is more preferably within the range of from 10° C. to 35° C., and even more preferably in the range of from 15° C. to 30° C.

The viscosity of the solution 25 coming out from the nozzle 13 is preferably in the range of from 1 mPa·s to 10 Pa·s. The viscosity of the solution 25 can be regulated by the temperature and the components of the solution 25. In a case in which the viscosity is regulated by the temperature of the solution 25, the temperature of the solution 25 may be regulated by the first temperature regulator 133. Furthermore, regarding a method for regulating the viscosity by means of the components of the solution 25, examples include a method of changing the concentration C of the cellulose acylate, and a method of changing the solvent. Regarding the method of changing the solvent, for example, in a case in which the solvent is composed of a single substance, the type of the single substance is changed, or the solvent is changed to a mixture by adding other components to the solvent. In a case in which the solvent is a mixture, at least any one of the components and the mixing ratio is changed. The viscosity of the solution 25 coming out through the nozzle 13 is more preferably in the range of from 1 mPa·s to 5 Pa·s, and even more preferably in the range of from 2 mPa·s to 2 Pa·s.

Similarly to the present embodiment, it is preferable that the nozzle 13 is provided with a cover 134 for covering the tip opening 13 a; and a second temperature regulator 135 for regulating the internal temperature of the cover 134. In the cover 134, an opening 134 a through which the solution 25 passes toward the collector 50, is formed between the tip opening 13 a and the collector 50. The atmosphere temperature Ta at the periphery of the tip opening 13 a (periphery of the outlet through which the solution comes out) is regulated by regulating the internal temperature by the second temperature regulator 135. The periphery is an area that covers at least the Taylor cone 44, and for example, it is preferable that the periphery is in the range of within 20 mm from the tip opening 13 a. It is preferable that the difference between the temperature Ts of the solution 25 coming out through the tip opening 13 a and the atmosphere temperature Ta, that is, Ts−Ta, is adjusted to the range of from −15° C. to 15° C. by regulating this atmosphere temperature Ta. In a case in which Ts−Ta is in the range of from −15° C. to 15° C., evaporation of the solvent occurs adequately compared to the case in which the difference is out of this range. Also, solidification of the solution 25 caused by drying is suppressed, and ball-shaped liquid droplets of the solution 25 and particles of solid components are not generated. The value of Ts−Ta is more preferably in the range of from −10° C. to 10° C., and even more preferably in the range of from −5° C. to 5° C.

The method of regulating the atmosphere temperature Ta at the periphery of the tip opening 13 a is not limited to the method involving the cover 134 and the second temperature regulator 135 of the present embodiment. Instead of the cover 134 and the second temperature regulator 135, for example, the atmosphere temperature Ta may also be regulated by sending a gas such as air that has been conditioned to have a constant temperature, to the spinning chamber 111, and regulating the temperature of the entire interior of the spinning chamber 111 by this transfer. In the present embodiment, the atmosphere temperature Ta is regulated to 25° C., and the relative humidity of the atmosphere at the periphery of the tip opening 13 a is adjusted to 30% RH.

The concentration C of the cellulose acylate in the solution 25 is preferably in the range of from 0.1 g/100 cm³ to 20 g/100 cm³. Thereby, the viscosity of the solution 25 becomes adequate, and the molecules of the cellulose acylate are appropriately entangled. The concentration C is more preferably from 0.5 g/100 cm³ to 15 g/100 cm³, and even more preferably from 1 g/100 cm³ to 10 g/100 cm³.

In the lower part of the nozzle 13, an accumulation unit 15 is disposed. The accumulation unit 15 includes a collector 50, a collector rotating unit 51, a support supply unit 52, and a support winding unit 53. The collector 50 is intended to capture the solution 25 coming out through the nozzle 13 as a nanofiber 46, and in the present embodiment, the solution 25 is captured on the support 60 that will be described below. The collector 50 is composed of a band-shaped endless belt made of a metal, for example, made of stainless steel. The collector 50 is not limited to be formed from stainless steel, and the collector 50 may be formed from any material that is charged by the power supply 65 by applying a voltage thereto. The collector rotating unit 51 is composed of a pair of rollers 55 and 56, and a motor 57. The collector 50 bridges horizontally over the pair of rollers 55 and 56. The shaft of one roller 55 is connected to the motor 57 disposed outside the spinning chamber 111, and thus the roller 55 is rotated at a predetermined speed. As a result of this rotation, the collector 50 moves so as to shuttle between the pair of rollers 55 and 56. In the present embodiment, the speed of movement of the collector 50 is 10 cm/hour; however, the invention is not limited to this.

To the collector 50, a support 60 formed from a band-shaped aluminum sheet (aluminum sheet) is supplied by the support supply unit 52. The support 60 according to the present embodiment has a thickness of approximately 25 μm. The support 60 is intended for accumulating (depositing) the nanofiber 46 thereon to obtain a nonwoven fabric 120. The support 60 on the collector 50 is wound by the support winding unit 53. The support supply unit 52 has a delivery shaft 52 a. A support roll 54 is mounted on the winding core 23 of the delivery shaft 52 a. The support roll 54 is configured to have the support 60 wound thereabout. The support winding unit 53 has a winding shaft 58. The winding shaft 58 is rotated by a motor that is not shown in the diagram and winds the support 60 on which the nonwoven fabric 120 has been formed, around the set winding core 61. The nonwoven fabric 120 is formed as a result of accumulation of the nanofiber 46. As such, this nanofiber producing apparatus 110 has a function of producing the nonwoven fabric 120 in addition to the function of producing the nanofiber 46. It is preferable that the speed of movement of the collector 50 and the speed of movement of the support 60 are adjusted to be the same so that no friction occurs between the two members. Furthermore, an embodiment in which the support 60 is mounted on the collector 50 and is caused to move along with the movement of the collector 50 is also acceptable.

The nonwoven fabric 120 may be formed by directly accumulating the nanofiber 46 on the collector 50; however, depending on the material forming the collector 50 or the surface state, there are nonwoven fabrics 120 that stick to the collector and are not easily detached. Therefore, as in the case of the present embodiment, it is preferable that the support 60 having the nonwoven fabric 120 stuck thereto with difficulties in detachment, is guided onto the collector 50, and the nanofiber 46 is accumulated on this support 60.

The power supply 65 is a voltage applying unit that applies a voltage to the nozzle 13 and the collector 50, charges the nozzle 13 to the first polarity, and charges the collector 50 to a second polarity, which is an opposite polarity of the first polarity. According to the present embodiment, the nozzle 13 is positively charged, and the collector 50 is negatively charged; however, the polarities of the nozzle 13 and the collector 50 may be reversed. The solution 25 is charged to the first polarity by passing through the nozzle 13. In the present embodiment, the voltage applied to the nozzle 13 and the collector 50 is set to 30 kV.

Regarding the distance L2 between the tip opening 13 a of the nozzle 13 and the collector 50, the appropriate value may vary depending on the types of the cellulose acylate and the solvent, the mass proportion of the solvent in the solution 25, and the like; however, the distance L2 is preferably in the range of from 30 mm to 300 mm, and in the present embodiment, the distance is set to 150 mm. In a case in which this distance L2 is 30 mm or more, a spinning jet 45 formed by jetting splits more reliably due to the repulsion caused by the electric charge of its own until the spinning jet 45 reaches the collector 50, compared to the case in which the distance L2 is shorter than 30 mm. Therefore, a finer nanofiber 46 can be obtained more reliably. Furthermore, as the nanofiber splits more finely as such, the solvent evaporates more reliably, and therefore, production of a nonwoven fabric containing residual solvent is prevented more reliably. Furthermore, in a case in which the distance L2 is 300 mm or less, the voltage to be applied can be supplied to a low level compared to the case in which the distance exceeds 300 mm and is too long, and therefore, abnormal discharge is suppressed.

[Nonwoven Fabric]

The nonwoven fabric of the invention is a nonwoven fabric formed from the nanofiber of the invention described above, and for example, as described above, the nonwoven fabric 120 can be produced by the nanofiber producing apparatus 110 illustrated in FIG. 1.

The nonwoven fabric of the invention can also be produced by detaching a deposit of the nanofiber obtained by an electrospinning method from the substrate, and heat-treating the deposit.

The contacting parts between nanofibers form strong bonding as a result of a curing reaction caused by heating, and thereby, a high-strength nonwoven fabric having excellent heat resistance and chemical resistance is obtained. The heating conditions are not particularly limited; however, conditions of heating for 10 minutes to 2 hours at 150° C. to 250° C. may be used.

The thickness of the nonwoven fabric of the invention can be adjusted as appropriate by the amount of depositing the nanofiber or by stacking nanofiber deposits each having an appropriate thickness, and the thickness is preferably about 30 nm to 1 mm, and more preferably about 100 nm to 300 μm.

The nonwoven fabric of the invention can be used for applications such as, for example, a medical filter, a face mask, a heat-resistant bag filter, a secondary battery separator, a secondary battery electrode, a heat insulating material, a filter cloth, and a sound absorbing material.

Among these, from the viewpoint that cellulose acylate has excellent biocompatibility, it is preferable to use the nonwoven fabric as a medical filter or a face mask. Furthermore, in the case of using the nonwoven fabric of the invention as a medical filter or a mask, it can be expected to have increased selective separation capacity. This is because since the nanofiber of the invention has high uniformity of the fiber diameter and high uniformity of porosity, the nanofiber exhibits excellent physical selective separation capacity, and cellulose acylate has the features of both hydrophilicity and hydrophobicity and exhibits high chemical selective separation capacity.

In the case of the heat-resistant bag filter, the nonwoven fabric can be used as a bag filter for use in general garbage incinerators and industrial waste incinerators.

Furthermore, in the case of the secondary battery separator, the nonwoven fabric can be used as a separator for use in lithium ion secondary batteries.

In the case of the secondary battery electrode, a deposit of a thermosetting nanofiber before thermosetting is used, and this can be used as a binder for forming a secondary battery electrode. Furthermore, an electrically conductive nonwoven fabric obtained by dispersing and mixing a powder electrode material into the spinning solution of the invention, electrospinning the mixture, and thermosetting a deposit obtained therefrom, can also be used as a secondary battery electrode.

In the case of the heat-insulating material, the nonwoven fabric can be used for a backup material for refractory bricks, or a combustion gas seal.

In the case of the filter cloth, the nonwoven fabric can be used as a filter cloth for microfilter, or the like by adjusting the thickness and the like of the nonwoven fabric as appropriate, and adjusting the pore size of the nonwoven fabric. By using a filter cloth, solid components in a fluid such as a liquid or a gas can be separated.

In the case of the sound absorbing material, the nonwoven fabric can be used as a sound absorbing material such as a wall surface sound insulation reinforcement or an inner wall sound absorbing layer.

EXAMPLES

Hereinafter, the invention will be described in more detail based on Examples. The materials, amounts used, proportions, treatments, treatment procedures, and the like disclosed in the following Examples can be modified as appropriate as long as the gist of the invention is maintained. Therefore, the scope of the invention should not be limitedly interpreted by the Examples described below.

Example 1

Cellulose (raw material: cotton linter) was mixed with an acylating agent and sulfuric acid as a catalyst, and acylation was carried out while the reaction temperature was maintained at 40° C. or lower. The acylating agent can be selected, as a single compound or a plurality of compounds, from acetic acid, acetic anhydride, propionic acid, propionic anhydride, butyric acid, and butyric anhydride depending on the intended degree of substitution. In Example 1, cellulose was acylated to have acetyl groups (in the following Table 1, abbreviated to “Ac”) using acetic acid.

After the raw material cellulose disappeared and acylation was completed, the system was further heated continuously at a temperature of 40° C. or lower, and the degree of polymerization was adjusted to a desired value.

Next, residual acid anhydride was hydrolyzed by adding an aqueous solution of acetic acid, and then partial hydrolysis was performed by heating at a temperature of 60° C. or lower. Thus, the degree of substitution was adjusted.

Residual sulfuric acid was neutralized with an excess amount of magnesium acetate. Reprecipitation from an aqueous solution of acetic acid was performed, and washing with water was repeated. Thus, a cellulose acylate was synthesized.

The cellulose acylate thus synthesized was dissolved in a mixed solvent of 90% of dichloromethane and 10% of N-methyl-2-pyrrolidone (NMP) to produce a cellulose acylate solution having a concentration of 4 g/100 cm³. A nonwoven fabric having a size of 20×30 cm, which was formed from cellulose acylate nanofibers, was produced using the nanofiber producing apparatus 110 illustrated in FIG. 1.

Examples 2 and 3

Nonwoven fabrics formed from nanofibers were produced in the same manner as in Example 1, except that the time for the partial hydrolysis was changed, and the degree of substitution based on acetyl groups was intentionally adjusted.

Example 4

A nonwoven fabric formed from nanofibers was produced in the same manner as in Example 1, except that the raw material cotton linter was subjected to an alkali purification treatment, and the hemicellulose amount was intentionally adjusted.

Example 5

A nonwoven fabric formed from nanofibers was produced in the same manner as in Example 1, except that the raw material was changed from cotton linter to hardwood pulp.

Examples 6 and 7

Nonwoven fabrics formed from nanofibers were produced in the same manner as in Example 1, except that the reaction time for acylation was changed, and the molecular weight was intentionally adjusted.

Example 8

A nonwoven fabric formed from nanofibers was produced in the same manner as in Example 1, except that the acyl group was changed from an acetyl group to a propionyl group (in the following Table 1, abbreviated to “Pr”).

Example 9

A nonwoven fabric formed from nanofibers was produced in the same manner as in Example 1, except that the acyl group was changed from an acetyl group to a butyryl group (in the following Table 1, abbreviated to “Bu”).

Comparative Examples 1 and 2

Nonwoven fabrics formed from nanofibers were produced in the same manner as in Example 1, except that the time for the partial hydrolysis was changed, and the degree of substitution based on acetyl groups was intentionally adjusted.

Comparative Example 3

A nonwoven fabric formed from nanofibers was produced in the same manner as in Example 8, except that the time for the partial hydrolysis was changed, and the degree of substitution based on propionyl groups was intentionally adjusted.

Comparative Example 4

A nonwoven fabric formed from nanofibers was produced in the same manner as in Example 9, except that the time for the partial hydrolysis was changed, and the degree of substitution based on butyryl groups was intentionally adjusted.

For the various cellulose acylates thus synthesized, the degree of substitution, the hemicellulose amount, the 6% solution viscosity, the number average molecular weight (Mn), the weight-average molecular weight (Mw), and the molecular weight distribution (Mw/Mn) were measured by the methods described above. The results are presented in the following Table 1.

<Evaluation>

Observations of the various nonwoven fabrics thus produced were made by visual inspection and using a scanning electron microscope (S-4300, magnification ratio: 1,800 times, manufactured by Hitachi, Ltd.), and the uniformity of the nonwoven fabrics was rated into five grades according to the following criteria. The results are presented in the following Table 1. Meanwhile, a nonwoven fabric having a score of 2 or higher can be put to practical use.

From SEM images of the various nonwoven fabrics, the average fiber length and the average fiber diameter of the nanofibers were measured by the methods described above, and the aspect ratio (average fiber length/average fiber diameter) was calculated from these values. The results are presented in the following Table 1.

SEM images obtained by making an observation of the nonwoven fabrics produced in Examples 1 and 2 and Comparative Example 1 are presented in FIG. 3 to FIG. 5, respectively.

Score 5: No defects are seen in any of the observations made by visual inspection and SEM.

Score 4: Defects are not seen by visual inspection; however, in the SEM images, some parts with non-uniform fiber diameters are observed.

Score 3: Defects are not seen by visual inspection; however, in the SEM images, many parts with non-uniform fiber diameters are observed.

Score 2: Some defects are seen by visual inspection, and in the SEM images, many parts with non-uniform fiber diameters are observed.

TABLE 1 Score 1: Many parts with non-uniform fiber diameters are observed in both visual inspection Nanofiber and SEM Hemi- 6% Average Average images. cellulose solution fiber fiber [Table 1] Cellulose raw Degree of amount viscosity Mw/ diameter length Aspect Uni- Table 1 material Substituent substitution mass % mPa · s Mn Mw Mn (mm) (mm) ratio formity Example 1 Cotton linter Ac 2.93 1.5 390 78,000 210,000 2.7 400 4.9 12,000 5 Example 2 Cotton linter Ac 2.87 1.3 390 78,000 210,000 2.7 530 3.1 5,800 4 Example 3 Cotton linter Ac 2.78 1.6 320 65,000 170,000 2.6 600 2.0 3,300 3 Example 4 Cotton linter Ac 2.83 0.05 380 78,000 200,000 2.6 650 2.3 3,500 3 (alkali treated) Example 5 Pulp Ac 2.85 3.5 370 62,000 200,000 3.2 620 1.7 2,700 3 Example 6 Cotton linter Ac 2.87 1.4 280 60,000 160,000 2.7 600 1.6 2,700 3 Example 7 Cotton linter Ac 2.87 1.5 150 42,000 110,000 2.6 760 1.0 1,300 2 Example 8 Cotton linter Pr 2.87 1.0 180 62,000 200,000 3.2 710 1.1 1,500 2 Example 9 Cotton linter Bu 2.86 1.3 110 55,000 175,000 3.2 780 1.0 1,300 2 Comparative Cotton linter Ac 2.43 1.2 220 81,000 210,000 2.6 3,300 0.4 120 1 Example 1 Comparative Cotton linter Ac 2.70 1.2 300 74,000 190,000 2.6 1,900 0.4 200 1 Example 2 Comparative Cotton linter Pr 2.46 1.0 150 63,000 200,000 3.2 4,600 0.3 80 1 Example 3 Comparative Cotton linter Bu 2.55 1.2 150 60,000 190,000 3.2 5,200 0.3 60 1 Example 4

From the results shown in Table 1, it was found that in a case in which a cellulose acylate having a degree of substitution of less than 2.75 was used, uniformity was poor irrespective of the type of the substituent, the hemicellulose amount, the 6% solution viscosity, and the like (Comparative Examples 1 to 4).

In contrast, it was found that in a case in which a cellulose acylate having a degree of substitution of from 2.75 to 2.95 was used, the nanofiber had excellent uniformity of the fiber diameter, and the nonwoven fabric acquired a satisfactory external appearance (Examples 1 to 9).

Particularly, from a comparison between Examples 2, 4, and 5, it was found that in a case in which the hemicellulose amount was in the range of 0.1 to 3.0, the nanofiber had more satisfactory uniformity of the fiber diameter, and the nonwoven fabric acquired a more satisfactory external appearance.

Furthermore, from a comparison between Examples 2, 6, and 7, it was found that in a case in which the 6% solution viscosity was 300 mPa·s or higher, the nanofiber had more satisfactory uniformity of the fiber diameter, and the nonwoven fabric acquired a more satisfactory external appearance.

From a comparison between Examples 1, 2, and 3, it was found that in a case in which the degree of substitution was 2.80 to 2.95, the nanofiber had more satisfactory uniformity of the fiber diameter, and the nonwoven fabric acquired a more satisfactory external appearance. It was also found that in a case in which the degree of substitution was 2.88 to 2.95, the nanofiber had more satisfactory uniformity of the fiber diameter, and the nonwoven fabric acquired a more satisfactory external appearance.

EXPLANATION OF REFERENCES

-   -   13: nozzle     -   13 a: tip opening     -   13 b: verge of tip opening     -   15: accumulation unit     -   23: winding core     -   25: solution     -   30: storage container     -   31: pump     -   32: pipe     -   44: Taylor cone     -   45: spinning jet     -   46: nanofiber     -   50: collector     -   51: collector rotating unit     -   52: support supply unit     -   52 a: delivery shaft     -   53: support winding unit     -   54: support roll     -   55: roller     -   56: roller     -   57: motor     -   58: winding shaft     -   60: support     -   61: winding core     -   65: power supply     -   110: nanofiber producing apparatus     -   111: spinning chamber     -   112: solution supply unit     -   120: nonwoven fabric     -   133: first temperature regulator     -   134: cover     -   134 a: opening     -   135: second temperature regulator     -   P: pump     -   M: motor     -   L2: distance 

What is claimed is:
 1. A nanofiber comprising a cellulose acylate having a degree of substitution that satisfies Formula (1): 2.75≤Degree of substitution≤2.95  (1)
 2. The nanofiber according to claim 1, wherein a proportion of an average fiber length with respect to an average fiber diameter is 1,000 or higher.
 3. The nanofiber according to claim 1, wherein an average fiber diameter is 50 to 800 nm.
 4. The nanofiber according to claim 1, wherein an average fiber length is 500 μm or more.
 5. The nanofiber according to claim 1, wherein an acyl group carried by the cellulose acylate is an acetyl group.
 6. The nanofiber according to claim 1, wherein a hemicellulose amount of the cellulose acylate is 0.1% to 3.0% by mass.
 7. The nanofiber according to claim 2, wherein a hemicellulose amount of the cellulose acylate is 0.1% to 3.0% by mass.
 8. The nanofiber according to claim 3, wherein a hemicellulose amount of the cellulose acylate is 0.1% to 3.0% by mass.
 9. The nanofiber according to claim 4, wherein a hemicellulose amount of the cellulose acylate is 0.1% to 3.0% by mass.
 10. The nanofiber according to claim 5, wherein a hemicellulose amount of the cellulose acylate is 0.1% to 3.0% by mass.
 11. The nanofiber according to claim 1, wherein a viscosity of a solution obtained by dissolving the nanofiber in dichloromethane to a concentration of 6% by mass is 300 mPa·s or higher.
 12. The nanofiber according to claim 2, wherein a viscosity of a solution obtained by dissolving the nanofiber in dichloromethane to a concentration of 6% by mass is 300 mPa·s or higher.
 13. The nanofiber according to claim 3, wherein a viscosity of a solution obtained by dissolving the nanofiber in dichloromethane to a concentration of 6% by mass is 300 mPa·s or higher.
 14. The nanofiber according to claim 4, wherein a viscosity of a solution obtained by dissolving the nanofiber in dichloromethane to a concentration of 6% by mass is 300 mPa·s or higher.
 15. The nanofiber according to claim 5, wherein a viscosity of a solution obtained by dissolving the nanofiber in dichloromethane to a concentration of 6% by mass is 300 mPa·s or higher.
 16. The nanofiber according to claim 6, wherein a viscosity of a solution obtained by dissolving the nanofiber in dichloromethane to a concentration of 6% by mass is 300 mPa·s or higher.
 17. A nonwoven fabric comprising the nanofiber according to claim
 1. 18. A nonwoven fabric comprising the nanofiber according to claim
 6. 19. A nonwoven fabric comprising the nanofiber according to claim
 11. 20. The nonwoven fabric according to claim 17, wherein the nonwoven fabric is used for a medical filter or a face mask. 